The present invention relates to optical characterization of a device under test. (DUT) by measurement of its optical return loss, especially its wavelength dependent optical return loss.
A measurement of optical return loss provides an optical signal with a certain incident optical power to the DUT and measures the fraction of the incident optical power that is reflected by the DUT. This fraction gives the optical return loss of the DUT. The measurement can be done while changing the wavelength of the optical signal to provide the optical return loss of the DUT as a function of the wavelength of the optical signal.
However, known measurement techniques (e.g. as disclosed in “Dennis Derickson, Fiber Optic Test and Measurement, Prentice Hall PTR, New Jersey, USA, 1998, pages 387-390”) show unwanted residual reflectivities of the measurement setup which limit the measurement range of such measurements to reflectivities of the DUT which are greater than the residual reflectivities.